In semiconductor manufacture, micro lithography is used in the formation of integrated circuits on a semiconductor wafer. During a lithographic process, a form of radiant energy such as ultraviolet light, is passed through a mask or reticle and onto the semiconductor wafer. The mask contains opaque and transparent regions formed in a predetermined pattern. A grating pattern, for example, may be used to define parallel spaced conducting lines on a semiconductor wafer. The ultraviolet light exposes the mask pattern on a layer of resist formed on the wafer. The resist is then developed for removing either the exposed portions of resist for a positive resist or the unexposed portions of resist for a negative resist. The patterned resist can then be used during a subsequent semiconductor fabrication process such as ion implantation or etching.
As microcircuit densities have increased, the size of the features of semiconductor devices have decreased to the sub micron level. These sub micron features may include the width and spacing of metal conducting lines or the size of various geometric features of active semiconductor devices. The requirement of sub micron features in semiconductor manufacture has necessitated the development of improved lithographic processes and systems. One such improved lithographic process is known as phase shift lithography.
With phase shift lithography the interference of light rays is used .to overcome diffraction and improve the resolution and depth of optical images projected onto a target. In phase shift lithography, the phase of an exposure light at the object is controlled such that adjacent bright areas are formed preferably 180 degrees out of phase with one another. Dark regions are thus produced between the bright areas by destructive interference even when diffraction would otherwise cause these areas to be lit. This technique improves total resolution at the object and allows resolutions as fine as 0.25 .mu.m to occur.
In general, a phase shifting photomask is constructed with a repetitive pattern formed of three distinct layers or areas. An opaque layer provides areas that allow no light transmission, a transparent layer provides areas which allow close to 100% of light to pass through and a phase shift layer provides areas which allow close to 100% of light to pass through but phase shifted 180 degrees from the light passing through the transparent areas. The transparent areas and phase shift areas are situated such that light rays diffracted through each area is canceled out in a darkened area there between. This creates the pattern of dark and bright areas which can be used to clearly delineate features of a pattern defined by the opaque layer of the mask on a photopatterned semiconductor wafer.
Recently, different techniques have been developed in the art for fabricating different types of phase shifting photomasks. One type of phase shifting mask, named after a pioneer researcher in the field, M. D. Levenson, is known in the art as a "Levenson" phase shifting mask. This type of mask is also referred to as an "alternating aperture" phase shifting mask because opaque light blockers alternate with light transmission apertures and every other aperture contains a phase shifter.
This type of mask is typically formed on a transparent quartz substrate. An opaque layer, formed of a material such as chromium, is deposited on the quartz substrate and etched with openings in a desired pattern. Phase shift areas on the mask are formed by depositing a phase shift material over the opaque layer and into every other opening in the opaque layer. This is termed an "additive" phase shifting mask. Alternately, phase shift areas of the mask may be formed in areas of the quartz substrate having a decreased thickness. This is termed a subtractive phase shifting mask.
Two types of Levenson phase shifting photomasks are shown in FIGS. 1A and 1B. FIG. 1A shows an additive phase shifting mask 8 comprising a transparent substrate 10 and an opaque layer 12 having a pattern of etched openings 16. The phase shifters 14 for the phase shifting mask 8 are formed as segments of a light transmissive material, such as SiO.sub.2, deposited in every other opening 16 in the opaque layer 12.
FIG. 1B shows a subtractive phase shifting mask 8A. In a subtractive phase shifting mask 8A, the phase shifters 14A are formed by etching the substrate 10A aligned with every other opening 16A in the opaque layer 12A. In the subtractive phase shifting mask 8A, the unetched portions of the substrate form the phase shifters 14A. Although the additive and subtractive phase shifting masks are fabricated by different methods, the operation of these photomasks is equivalent.
Another type of phase shifting photomask is known as a rim phase shifting mask. A rim phase shifting mask includes phase shifters that are formed on the edges of the opaque light blocking elements. U.S. Pat. No. 5,194,345 to J. Brett Rolfson, also the present Applicant, discloses a rim phase shifting photomask. A rim phase shifting mask 18, fabricated in accordance with the '345 patent, is shown in FIG. 2. The phase shifting mask 18 includes a transparent substrate 20 having a pattern of opaque light blockers 22 formed thereon. Between each opaque light blocker 22 is a light transmission opening 24. The pattern formed by the opaque light blockers 22 and the light transmission openings 24, establishes the image that is projected by the phase shifting mask 18 onto a wafer during a photolithography process.
In the rim phase shifting mask 18, a layer of a transparent phase shift material 26, such as SiO.sub.2, is conformally deposited over the opaque light blockers 22 and into the light transmission openings 24. This produces rim phase shifters 28 on the sidewalls of the opaque light blockers 22 on either side of each light transmission opening 24. In use of the phase shifting mask 18, the light passing through a rim phase shifter 28 is phase shifted relative to the light passing through a light transmission opening 24. This is because the light passing through a rim phase shifter 28 must pass through a thicker section of the transparent phase shift material 26. The phase shifted light forms a null on the wafer that corresponds to the edges of the opaque light blockers 22. This overcomes the effects of diffraction along the edges of the opaque light blockers 22 and produces a sharpened image.
In the '345 patent, the thickness "t" of the opaque light blockers 22 is selected to form rim phase shifters 28 that achieve a phase shift of 180.degree. (.pi.) or an odd whole multiple of 180.degree.. This thickness "t" can be determined using the formula: EQU t=i.lambda./2(n-1)
where
t=thickness of opaque light blockers PA1 i=an odd integer PA1 .lambda.=wavelength of exposure light PA1 n=refractive index of phase shift material
at the exposure wavelength.
Although this is an effective method for establishing the thickness of a rim phase shifter, it may be difficult to form the opaque layer to the required thickness (t) without employing custom fabrication processes. Specifically, standard mask blanks employed in semiconductor manufacture, are difficult to use with this method because the opaque layer on these masks is not formed to the required thickness "t".
Standard mask blanks are manufactured and sold by mask makers for use in semiconductor manufacture. These mask blanks include a transparent substrate and an opaque layer deposited and planarized to a standard thickness (e.g., 800-1200 .ANG.). The opaque layer is then patterned and etched with a particular integrated circuit layout for use in the semiconductor manufacturing process. The thickness of the opaque layer on a standard mask blank, is established without regard to achieving a phase shift, as taught in the '345 patent.
Because it is difficult to modify the existing opaque layer of a standard mask blank to a different thickness, a standard mask blank can not be used in forming a phase shifting mask under the method disclosed in the '345 patent. Accordingly this type of phase shifting mask must be custom fabricated. This may be an expensive and time consuming process.
In view of the foregoing it is an object of the present invention to provide an improved method for fabricating rim phase shifting masks for semiconductor manufacture. It is a further object of the present invention to provide an improved method for fabricating rim phase shifting masks in which standard mask blanks can be employed to facilitate the fabrication process. It is yet another object of the present invention to provide an improved method for fabricating rim phase shifting mask that is simple, low cost and adaptable to large scale semiconductor manufacture.